Device for generating a video image sharpness improvement signal

ABSTRACT

A device for producing a video image sharpness improvement signal (DOC 21 -DOC 22 ), with black level clipping of an associated video signal, comprises a differential transconductance stage processing the video signal and whose bias currents (Imax) are directly proportional to the active component (ΔV) of the video signal so as to bring about a black level sharpness improvement signal clipping.

FIELD OF THE INVENTION

The present invention relates to a device for generating a video imagesharpness improvement signal, and more generally to the implementationof a sharpness improvement function and of a black level clippingfunction.

It pertains to the field of the displaying of still or moving images onthe screen of a television or of a computer monitor, whether the screenbe a cathode ray tube or CRT screen, a liquid crystal display or LCD, aplasma screen, or the like.

The invention is described hereinbelow in the case of a CRT screen,although this is not limiting. For such a screen, the invention may forexample be implemented in the video preamplifier associated with thescreen.

BACKGROUND OF THE INVENTION

Represented diagrammatically in FIG. 1 a is a portion of an input videosignal VIDEO_IN, corresponding to the displaying of an image line on thescreen. This portion lies between two line-blanking intervals 11 and 12designed for the flyback of the beam deflection armatures of the cathoderay tube. For the sake of simplicity, the video signal is considered toexhibit the form of a rectangular voltage pulse 13 inside the videosignal portion considered.

Represented in FIG. 1 b is an image improvement control signal PBC. Thissignal is synchronized with the signal VIDEO_IN. For example, the signalPBC is at the high level when the image is to be improved, and at thelow level otherwise. In the example shown in the figure, the image isthus improved within a zone situated substantially at the centre of theline which corresponds to the portion of the signal VIDEO_IN between theintervals 11 and 12. This zone corresponds to a rectangular pulse 14 ofthe signal PBC.

Represented in FIG. 1 c is a signal VIDEO_OUT at the output of a circuitwhich receives the signal VIDEO_IN as input, and which implements theimage improvement functions. In the example considered here, thiscircuit is for example the screen's video preamplifier.

Outside the improved zone of the image, the signal VIDEO_OUT correspondsto the signal VIDEO_IN. Inside this zone, however, a pulse 13′corresponding in the signal VIDEO_OUT to the pulse 13 in the signalVIDEO_IN differs from said pulse through the following characteristics:

the low level of the pulse 13′ is higher than that of the pulse 13, thisresulting from the image brightness improvement function;

the difference between the high level and the low level of the pulse 13′is higher than for the pulse 13, this resulting from the image contrastimprovement function;

the pulse 13′ exhibits a positive peak 15 (an overvoltage) subsequent toits rising edge and a negative peak 16 subsequent to its falling edge,which result from the image sharpness (vivacity) improvement function.In the example represented, these peaks have exponential decay; and

the positive peak 16 of the pulse 13′ is clipped at the minimum level ofthe signal VIDEO_IN, this resulting from the black level clippingfunction.

The positive peak 15 participates in the sought-after sharpnessimprovement, which is dependent both on the amplitude and on the timeconstant of the peak 15. This is why it is also referred to as the“sharpness peak” in the jargon of the person skilled in the art and inwhat follows. The negative peak 16 also participates in the sharpnessimprovement. As is known in the state of the art, the negative peak 16is nevertheless clipped at the black level, which corresponds to a zerovalue of the active component of the signal VIDEO_IN, so as not todisturb the operation of the display system.

In the state of the art, various ways of carrying out the sharpnessimprovement function are known. Apart from the entirely digital methods,methods involving differentiation of the input video signal and methodsusing a delay line are thus known.

The time charts of FIGS. 2 a to 2 c illustrate the principle of themethods involving differentiation of the signal VIDEO_IN. Forsimplicity, the signal VIDEO_IN (FIG. 2 a) considered here is arectangular pulse. By differentiation, one firstly obtains thedifferentiated signal VIDEO_IN (FIG. 2 b), which exhibits a positivepeak and a negative peak subsequent respectively to the rising edge andto the falling edge of the signal VIDEO_IN. Next by adding the signalVIDEO_IN and the differentiated signal VIDEO_IN, one obtains the outputvideo signal VIDEO_OUT (FIG. 2 c).

The time charts of FIGS. 3 a to 3 d illustrate the principle of themethods using a delay line. For simplicity, the signal VIDEO_IN (FIG. 3a) considered here is also a rectangular pulse. By passing the signalVIDEO_IN through a delay line, one firstly obtains the delayed signalVIDEO_IN (FIG. 3 b). Next by differencing the signal VIDEO_IN and thedelayed signal VIDEO_IN, one obtains a signal (FIG. 3 c) comprising apositive peak and a negative peak subsequent respectively to the risingedge and to the falling edge of the signal VIDEO_IN. Then by adding thissignal to the signal VIDEO_IN, one obtains the signal VIDEO_OUT (FIG. 3d).

However, the known implementations of these techniques bydifferentiation or by delay lines are not satisfactory in that they donot allow easy adjustment of the amplitude and of the time constant ofthe sharpness peak, and/or do not allow black level clipping, and/or areexpensive to implement (in particular in the case of delay linetechniques).

BRIEF SUMMARY OF THE INVENTION

An object of the invention consists in proposing an original solutionfor simultaneously implementing the sharpness improvement function andblack level clipping function.

This aim is achieved, according to a first aspect of the invention, byvirtue of a device for producing a sharpness improvement signal for avideo image with black level clipping of an associated video signal,said device comprising:

a first differential transconductance stage comprising:

a voltage input for receiving the video signal, said video signalcomprising a determined fixed component which corresponds to the blacklevel and a variable active component;

a differential pair with a first transistor having a control terminalcoupled to the voltage input, and a second transistor having a controlterminal coupled to the voltage input via an integrator;

a first controlled current source coupled to a first main terminal ofthe first transistor and a second controlled current source coupled to afirst main terminal of the second transistor, said first current sourceand said second current source each delivering one and the same variablebias current, a resistor connected between said first respective mainterminals of the first and of the second transistor;

a first current output coupled to a second main terminal of the firsttransistor and a second current output coupled to a second main terminalof the second transistor, the sharpness improvement signal being thedifferential current signal between said first and second currentoutputs; and

means for producing a control signal for the first and for the secondcurrent source, in such a way that the bias currents are directlyproportional to the active component of the video signal.

Such a sharpness improvement signal is intended to be added to the videosignal. The maximum amplitude of the sharpness peak is determined by thevalue of the resistor. Its time constant is determined by the integratorcircuit. Furthermore, as will be more clearly apparent in what follows,the fact that the bias currents of the transconductance stage aredirectly proportional to the active component of the video signal bringsabout a clipping of the sharpness improvement signal at the black level.

It will noted that the sign of the differential current between thefirst and second current outputs of the first differentialtransconductance stage is immaterial, this being manifested by thesymmetry between the positive peak 15 and the negative peak 16 (FIG. 1c).

A second aspect of the invention pertains to a video preamplificationcircuit comprising a device according to the first aspect.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Other characteristics and advantages of the invention will becomefurther apparent on reading the description which follows. The latter ispurely illustrative and should be read in conjunction with the appendeddrawings in which:

FIGS. 1 a-1 c, are time charts illustrating various known functions forimproving a video image;

FIGS. 2 a-2 c, are time charts illustrating a known technique forimplementing the differentiation-based sharpness improvement function;

FIGS. 3 a-3 d, are time charts illustrating a known technique forimplementing the delay line-based sharpness improvement function;

FIG. 4 is a schematic diagram of a circuit for processing a video signalincorporating a device according to the invention;

FIGS. 5 a-5 d are time charts illustrating a technique of implementingthe integration-based sharpness improvement function, according to theinvention;

FIG. 6 is a simplified diagram of one embodiment of a device accordingto the invention;

FIGS. 7 a-7 c are time charts illustrating a sharpness improvementsignal produced by a device according to FIG. 6;

FIG. 8 is a simplified diagram of one control stage for the deviceaccording to FIG. 6; and,

FIGS. 9 a-9 c are time charts illustrating a sharpness improvementsignal produced by a device according to FIG. 6, with furthermore blacklevel clipping by virtue of the control by a stage according to FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

In certain applications, in particular for computers (screen of acomputer monitor), it is desirable to improve the quality of the displayfor certain screen zones in which still or moving images are displayed,with respect to other zones of the screen where text data or icons aredisplayed. For this purpose, display parameters such as the contrast,the brightness and the sharpness or vivacity of the image are modified.In particular, for the displaying of photographs or film sequences, itis preferable to increase the values of the contrast, brightness and/orsharpness in order to ensure better photographic rendition. It will benoted that the values of these parameters should be increased only inthe zones in question, so as not to make it difficult or tiring for theeyes to read the text data or icons displayed in the remainder of thescreen.

FIG. 4 is a schematic diagram of one embodiment for processing a videosignal VIDEO_IN, implementing the image improvement functions presentedin the introduction, with regard to FIGS. 1 a-1 c. Such a device is forexample included in a video preamplification circuit which is generallyarranged just upstream of the screen.

The circuit comprises an input 1, for receiving the video signalVIDEO_IN to be processed. This signal has for example the profilerepresented in FIG. 1 a. It is typically an R, G or B signal for anapplication to a computer monitor fitted with a CRT screen.

The circuit can also comprise one or more inputs such as the input 2,for receiving commands for adjusting the brightness, contrast, and/orsharpness (amplitude and/or time constant of the sharpness peak).

The circuit further comprises an input 3, for receiving an imageimprovement control signal PBC. This signal typically has the profilerepresented in FIG. 1 b.

Finally the circuit comprises an output 4 for delivering the outputvideo signal VIDEO_OUT. This signal has for example the profilerepresented in FIG. 1 c.

The circuit comprises an input stage 10, a stage for generating asharpness improvement signal 20, a stage for generating a contrastimprovement signal 30, a stage for generating a brightness improvementsignal 40, and a video amplification stage or output stage 50. Thestages 10 to 30 are stages of transconductance type (voltage/currentconverters). The stage 50 is a transimpedance stage (current/voltageconverter). The inputs of all these stages are designated by the letterX followed by a number, and their outputs are designated by the letter Ufollowed by a number.

Thus, the stage 10 comprises a voltage input X11, two differentialcurrent outputs U11 and U12, and two other differential current outputsU13 and U14. The stage 20 comprises a voltage input X21, a control inputX22 and two differential current inputs X23 and X24. The stage 30comprises a voltage input X31, a control input X32, two differentialcurrent outputs U31 and U32, and two other differential current outputsU33 and U34. The stage 40 comprises a control input X41, twodifferential current outputs U41 and U42, and two other differentialcurrent outputs U43 and U44. And the stage 50 comprises two differentialcurrent inputs X51 and X52, and a voltage output U51.

The output U51 of the stage 50 is coupled to the output 4 of thecircuit, to deliver the signal VIDEO_OUT.

The inputs X11, X21 and X31 of the stages 10, 20 and 30 respectively,are coupled to the input 1 of the circuit for receiving the signalVIDEO_IN. The inputs X22, X32 and X41, of the stages 20, 30 and 40respectively, are coupled to the input 2 of the circuit for receiving anadjustment command. For the sake of simplicity, a single control input 2is represented, this control being common to the aforesaid stages.However, it is of course understood that the circuit can compriseseveral such control inputs, in particular at least one for each stagerespectively, so that the sharpness (amplitude and/or time constant ofthe sharpness peak), the contrast and the brightness may be adjustedindependently of one another.

The inputs X23 and X24 of the stage 20 are respectively coupled to theoutputs U13 and U14 of the stage 10, and also to the outputs U33 and U34of the stage 30, and also to the outputs U43 and U44 of the stage 40. Aswill be set forth in greater detail hereinbelow, the inputs X23 and X24of the stage 20 thus receive a control signal for a bias current, in theform of a differential current, an additive contribution of which isafforded by each of the stages 10, 30 and 40 respectively.

The outputs U11 and U12 of the stage 10 are respectively linked to theinputs X51 and X52 of the stage 50. The outputs U21 and U22 of the stage20, the outputs U31 and U32 of the stage 30, and the outputs U41 and U42of the stage 40, are respectively linked to the inputs X51 and X52 ofthe stage 50 via a two-way switch 60. In an open position of the switch60, the inputs X51 and X52 of the stage 50 receive only the currentsdelivered by the outputs U11 and U12 respectively of the stage 10.However in a closed position of the switch 60, they receive, additively,the currents delivered by the outputs U11 and U12 respectively of thestage 10, the outputs U21 and U22 respectively of the stage 20, theoutputs U31 and U32 respectively of the stage 30, and the outputs U41and U42 respectively of the stage 40. A control input of the switch 60is linked to the input 3 of the circuit for receiving the imageimprovement control signal PBC, so that its open or closed position iscontrolled by this signal. In one example corresponding to the presentembodiment, the closed position of the switch 60 (respectively open) isobtained for a signal PBC in the high state (respectively low state).

As will have been understood, there are several paths for processing thesignal VIDEO_IN. A main path or serial path, corresponding to the openposition of the switch 60 (that is with no image improvement functions),passes through the stages 10 and 50 only. Three parallel paths,corresponding to the closed position of the switch 60 (that with imageimprovement functions), add the processing of the stages 20, 30 and 40to the processing performed by the stage 10 of the main path, the wholebeing subjected to the processing of the stage 50.

The fact that each image improvement function, namely the improvement ofthe sharpness, the improvement of the contrast and the improvement ofthe brightness, is carried out by a differential transconductance stage20, 30 and 40 respectively, connected in parallel with the main path,affords a certain number of advantages. Specifically, these stages 20,30 and 40 add a differential signal at the input of the stage 50 onlywhen the image improvement functions are active (signal PBC in the highstate).

The parallel configuration makes it possible in particular not toincrease the noise on the main path, with respect to a seriesarrangement of stages working on voltages.

Moreover, this parallel configuration makes it possible to minimize theconsumption of current and hence the heat dissipation problems.Specifically, the supply to the stages 20, 30 and 40 can be completelycut when the signal PBC is in the low state, while a series arrangementof stages working on voltages would require at the minimum themaintaining of a bias voltage of these stages.

A third advantage is related to the differential structure of the stages20, 30 and 40, and resides in the good performance in terms of noisemade possible by this structure.

In what follows, only the operation of the stage 20 is described indetail, and that of stage 10 is described only as far as is necessary tounderstand the operation of stage 20. Specifically, the inventionpertains solely to the implementation of the sharpness improvementfunction, and the description of the implementation of the other imageimprovement functions is not required here.

The time charts of FIGS. 5 a-5 d illustrate the principle of theimplementation of the sharpness improvement function according to theinvention. This principle and these time charts should be compared withwhat was presented in the introduction with regard to FIGS. 2 a-2 c and3 a-3 d.

For the sake of simplicity, the signal VIDEO_IN is here again consideredin a portion in which it exhibits the form of a rectangular voltagepulse, as is visible in FIG. 5 a. The principle of the inventionconsists in integrating the signal VIDEO_IN so as to obtain theintegrated signal VIDEO_IN which is represented in FIG. 5 b. Then, thedifference is computed between the signal VIDEO_IN and the integratedsignal VIDEO_IN, so as to obtain a sharpness improvement signal asrepresented in FIG. 5 c. Finally, the addition of the sharpnessimprovement signal to the signal VIDEO_IN gives the signal representedin FIG. 5 d, this corresponding to the signal VIDEO_OUT without theaddition of contrast and brightness improvement signals.

The device according to the invention, one embodiment of which will nowbe described, has as function to produce a sharpness improvement signalas represented in FIG. 5 c, with, further, the implementation of a blacklevel clipping function.

The diagram of FIG. 6 illustrates this embodiment. Stage 20 which isrepresented therein comprises a differential pair arranged between thevoltage input X21, which receives the signal VIDEO_IN, and the currentoutputs U21 and U22, which deliver currents DOC21 and DOC22respectively, the difference of which corresponds to the sharpnessimprovement signal.

This differential pair comprises a transistor T21 and a transistor T22,for example bipolar transistors, and in particular of PNP type. Thetransistors T21 and T22 have a control electrode (base) coupled to theinput X21. The base of T22 is coupled to the input X21 via an integratorINT.

The integrator INT comprises for example a cell RC with a resistor inseries R followed by a capacitor C in parallel with an earth terminal.

A first main electrode (emitter) of the transistors T21 and T22 iscoupled to a positive supply terminal delivering a voltage Vcc, via acontrolled current source, SC21 and SC22 respectively. These currentsources each deliver one and the same current Imax. The currents Imaxbias the differential pair.

A resistor Rtrans is connected between the respective emitters of thetransistors T21 and T22.

The outputs U21 and U22 are coupled to the second main terminal(collector) of the transistors T21 and T22 respectively.

Of course, the embodiment of the stage 20 described hereinabove is notlimiting. In particular, a dual embodiment based on a pair of NPN typebipolar transistors is conceivable. Likewise, embodiments based on apair of MOS transistors, either of N type or of P type, are alsoconceivable. These, and other embodiments may readily be deduced by aperson skilled in the art from the foregoing.

The operation of the stage 20 will now be described in conjunction withthe time charts of FIGS. 7 a-7 c. Initially, the assumption is made thatthe current Imax is constant.

Represented in FIG. 7 a is a portion of the signal VIDEO_IN. For thesake of simplicity, it is assumed that in this portion the signalVIDEO_IN exhibits the profile of a rectangular voltage pulse, between azero voltage and a determined voltage V1.

Represented in FIG. 7 b is the corresponding profile of the currentsDOC21 and DOC22 delivered by the current outputs U21 and U22respectively of the stage 20, in a first case where the current Imax isgreater than V1/Rtrans.

At equilibrium, that is for a stable voltage VIDEO_IN, the current ineach branch of the differential pair is equal to Imax, that is DOC1=DOC2=Imax. Through the effect of the integrator INT, the current DOC21exhibits however a negative peak on the rising edge of the signalVIDEO_IN and a positive peak on the falling edge of the signal.Conversely, the current DOC22 exhibits a positive peak on the risingedge of the signal VIDEO_IN and a negative peak on the falling edge ofthis signal. The amplitude of these peaks is equal to Imax±V1/Rtrans.These are exponentially decaying peaks, with a time constant equal toR×C.

The sharpness improvement signal, which consists of the differencebetween the differential currents DOC22 and DOC21, therefore exhibits apositive peak of maximum amplitude equal to 2×V1/Rtrans on the risingedge of the signal VIDEO_IN (corresponding to the sharpness peak) and anegative peak of maximum amplitude equal to −2×V1/Rtrans on the fallingedge of the signal VIDEO_IN. Advantageously, the amplitude of thesharpness peak is therefore controlled solely by the value of theresistor Rtrans. And its time constant is determined by the value of thehardware items R and C of the integrator INT.

Advantageously, a command for adjusting the maximum amplitude or thetime constant of the sharpness peak can therefore act respectively onthe value of Rtrans (for the maximum amplitude), or on the value of Rand/or of C (for the time constant). Such a command is for examplereceived on the adjustment control input X22 of the stage 20, whichinput is not represented on the diagram of FIG. 6 so as not tooverburden this figure.

Represented in FIG. 7 c is the corresponding profile of the currentsDOC21 and DOC22, in a second case where the current Imax is less thanthe V1/Rtrans.

At equilibrium, we still have the relation DOC1=DOC2=Imax. However, thecurrent in the differential pair being limited by design to 2×Imax, itis noted that here the currents DOC21 and DOC22 are clipped atImax±Imax, that at 0 for the negative peaks and at 2×Imax for thepositive peaks. Stated otherwise, the sharpness improvement signal,which corresponds to the difference between the differential currentsDOC22 and DOC21, therefore exhibits a positive peak of maximum amplitudelimited to 2×Imax on the rising edge of the signal VIDEO_IN and anegative peak of amplitude limited to −2×Imax on the falling edge of thesignal VIDEO_IN.

Advantageously, the invention uses this natural clipping effect for theimplementation of the clipping of the video signal at the black level.For this purpose, the invention proposes that the value of the biascurrent Imax of the stage 20 be controlled, as a function of the activecomponent of the signal VIDEO_IN. More exactly, the current Imax iscontrolled in a manner proportional to the active component of thesignal VIDEO_IN, so that there is no clipping in respect of the positivepeaks and that there is conversely black level clipping in respect ofthe negative peaks of the sharpness improvement signal.

This implementation will now be described in detail with regard to thediagram of FIG. 8.

Before tackling this description, it is appropriate firstly to note thatthe black level does not correspond, in general, to a zero value of thesignal VIDEO_IN. Specifically, for reasons of bias of the electronicprocessing circuits, this signal comprises a non-zero fixed component,called Vref in what follows, which corresponds to the black level and anactive component called ΔV in what follows, which carries the videoinformation proper. For simplicity, only the active component of thesignal VIDEO_IN has been taken into account in the foregoing. Statedotherwise, in the foregoing it was considered that the fixed componentVref of the signal VIDEO_IN was zero, this not being the case inpractice. Typically, Vref is of the order of 1.0 volts and ΔV variesbetween 0 and 1.0 volts (peak-to-peak).

In what follows, we consider a signal VIDEO_IN such that:VIDEO_IN=Vref+ΔV   (1)

where Vref designates the fixed component of the signal VIDEO_IN, and

where ΔV designates the active component of the signal VIDEO_IN.

Represented diagrammatically in FIG. 8 is one embodiment of the stage10.

This stage comprises, between the voltage input X11 and the currentoutputs U11 and U12, a differential pair with a first transistor T11 anda second transistor T12. It also comprises, between the input X11 andthe current outputs U13 and U14, another differential pair with a firsttransistor T13 and a second transistor T14.

The transistors T11-T14 are for example bipolar transistors, inparticular of PNP type. They each comprise a control electrode (base)coupled to the input X11, for receiving the signal VIDEO_IN. A firstmain electrode (emitter) of the transistors T11 and T13 is coupled to apositive supply terminal delivering the voltage Vcc, via a controlledcurrent source SC11. Likewise, the emitter of the transistors T12 andT14 is coupled to said positive supply terminal, via a controlledcurrent source SC12. The current sources SC11 and SC12 each deliver oneand the same current Ibias. The currents Ibias bias the differentialpairs T11-T12 and T13-T14.

A resistor Rsignal is connected between the emitters of the transistorsT11 and T13 on the one hand, and the emitters of the transistors T12 andT14 on the other hand. The value of this resistor Rsignal is less thanor equal to the resistance Rtrans of stage 20. Advantageously, provisionmay be made for Rsignal≦Rtrans, preferably${{Rsignal} \leq {\frac{1}{2} \times {Rtrans}}},$for example ${Rsignal} = {\frac{1}{2} \times {Rtrans}}$

The outputs U11 to U12 are coupled to the second main terminal(collector) of the transistors T11 and T12 respectively. Likewise, theoutputs U13 and U14 are coupled to the collector of the transistors T13and T14 respectively.

Of course, the embodiment of stage 10 described hereinabove is notlimiting. In particular, a dual embodiment based on a pair of NPN-typebipolar transistors is conceivable. Likewise, embodiments based on apair of MOS transistors, either of N type or P type, are alsoconceivable. These other embodiments may readily be deduced by theperson skilled in the art from the foregoing.

The outputs U11 and U12 deliver currents DOC11 and DOC12 respectively.The differential current between the outputs U11 and U12 is equal to thedifference between the currents DOC11 and DOC12. If the assumption ismade that the currents Ibias distribute equitably between thetransistors T11 and T13 (i.e., in the absence of offset), then thisdifferential current is equal to$\frac{Ibias}{2} \pm {\frac{\Delta\quad V}{2 \times {Rsignal}}.}$Its maximum amplitude is therefore equal to$\frac{\Delta\quad V}{Rsignal}.$

Likewise, the outputs U13 and U14 deliver currents DOC13 and DOC14respectively. The differential current between the outputs U13 and U14is equal to the difference between the currents DOC13 and DOC14. If thesame assumption as above is made, then the differential current is alsoequal to${\frac{Ibias}{2} \pm \frac{\Delta\quad V}{2 \times {Rsignal}}},$and its maximum amplitude is also equal to$\frac{\Delta\quad V}{Rsignal}.$

In practice, it may be necessary to obtain differential currents betweenthe outputs U11 and U12 on the one hand, and between the outputs U13 andU14 on the other hand, which have different respective maximumamplitudes.

This result may be achieved by placing an unequal number of bipolartransistors in parallel with the transistors T11 and T12 on the onehand, and with the transistors T13 and T14 on the other hand. Thisamounts to multiplying the currents DOC11 and DOC12 on the one hand, andthe currents DOC13 and DOC14 on the other hand, by nonidenticalcoefficients.

When the transistors T11-T14 are MOS transistors, it is possible toobtain a result of the same nature (introduction of a ratio N betweenthe currents DOC11 and DOC12 on the one hand, and the currents DOC13 andDOC14 on the other hand), by providing for a ratio N between the size(channel width) of the transistors T11 and T12 on the one hand, and thesize of the transistors T13 and T14 on the other hand.

The stage 10 is coupled to stage 20, for example via a current mirrorarrangement (not represented), in such a way that the following relationholds: ${Imax} = {\frac{1}{2} \times \Delta\quad{V/{Rsignal}}}$

Stated otherwise, there is provision for the bias current Imax of thedifferential pair T21-T22 of stage 20 to be proportional to the activecomponent ΔV of the signal VIDEO_IN, and, more particularly, equal tohalf of the differential current signal between the current outputs U11and U12 of the stage 10.

This current mirror arrangement can be integrated with stage 20, forexample. For example, it may take the form of a transimpedance stage,having two differential current inputs linked to the current inputs X23and X24 of the stage 20 (see FIG. 4), and a voltage output that controlsthe current sources SC21 and SC22 of the stage 20, when these sourcesare voltage controlled. The embodying of such an assembly is within thescope of the person skilled in the art and will not be detailed here.

The operation of stage 20, when it is coupled to stage 10 in the mannerjust described, will now be described in conjunction with the timecharts of FIGS. 9 a-9 c.

Represented in FIG. 9 a, which should be compared with the time chartsof FIGS. 2 a, 3 a and 7 a, is the active component ΔV of the signalVIDEO_IN for a determined portion of this signal.

For the sake of simplicity, it is assumed that in this portion thesignal VIDEO_IN exhibits the profile of a rising voltage step, betweenthe voltage Vref corresponding to the black level and a determinedvoltage Vref+V1 followed by a falling voltage step, between said voltageVref+V1 and another determined voltage Vref+V2, where V2<V1. Statedotherwise, the component ΔV exhibits the values 0, V1 and V2successively.

Represented in FIG. 9 b is the corresponding profile of the currentsDOC21 and DOC22 delivered by the current outputs U21 and U22respectively of stage 20. Represented in FIG. 9 c is the sharpnessimprovement signal, which corresponds to the difference between thesetwo currents, namely the differential current DOC22-DOC21. The curves ofFIG. 9 b are to be compared with those of FIGS. 7 b and 7 c, bydistinguishing three time domains that are labelled in the figure bynumbers 1 to 3 in small circles, and which are associated with the threevalues of the current Imax(ΔV), namely Imax(0), then Imax(V1), andfinally Imax(V2).

Firstly, when ΔV=0, the current Imax is equal to Imax(0), which is equalto 0. The currents DOC21 and DOC22 are equal to Imax(0), thedifferential pair T21-T22 being balanced, and their differenceDOC22-DOC21 is zero.

Secondly, when ΔV=V1, the current Imax(V1) is equal to V1/(2×Rsignal).At the time of the first voltage step, the currents DOC21 and DOC22exhibit a peak (with exponential decay) respectively negative andpositive, the maximum amplitude of which is equal to Imax(V1)±V1/Rtransand the time constant of which is equal to R×C, since the differentialpair T21-T22 is unbalanced by the rising edge of the signal VIDEO_IN.The difference DOC22-DOC21 between the currents DOC22 and DOC21 isnon-zero. In fact this difference is a positive peak whose maximumamplitude is equal to 2×V1/Rtrans, and whose time constant is equal toR×C. The condition V1/Rtrans≦Imax(V1) that V1/Rtrans≦V1/Rsignal beingsatisfied with${{{Imax}({V1})} = {{\frac{1}{2} \times {{V1}/{Rsignal}}\quad{and}\quad{Rsignal}} \leq {\frac{1}{2} \times {Rtrans}}}},$there is no clipping.

Stated otherwise, it is by providing that the bias currents Imax of thestage 20 be greater than the input signal's active component convertedinto current (that ΔV/Rtrans) that the absence of clipping isguaranteed. This arrangement is satisfactory when${I\quad\max} = {{\frac{1}{2} \times \Delta\quad V\text{/}{Rsignal}\quad{and}\quad{when}\quad{Rsignal}} \leq {\frac{1}{2} \times {{Rtrans}.}}}$

Thirdly, when ΔV=V2, the current Imax(V2) is equal to V2/(2×Rsignal). Atthe time of the second voltage step, the differential pair T21-T22 isunbalanced by the falling edge of the signal VIDEO_IN, so that thecurrents DOC21 and DOC22 exhibit a peak respectively positive andnegative, the maximum amplitude of which is equal toImax(V2)±(V1-V2)/Rtrans and the time constant of which is equal to$\frac{1}{2} \times R \times {C.}$The difference DOC22-DOC21 between the currents DOC22 and DOC21 isnon-zero. This difference ought to correspond to a negative peak whosemaximum amplitude would be equal to −2×(V1-V2)/Rtrans. Nevertheless, thecondition (V1-V2)/Rtrans≦Imax(V2) not being satisfied with${{I\quad{\max({V2})}} = {{\frac{1}{2} \times {{V2}/{Rsignal}}\quad{and}\quad{Rsignal}} = {\frac{1}{2} \times {Rtrans}}}},$there is clipping of the currents DOC22 and DOC21, so that theirdifference is at most equal to −2×Imax(V2) that to −V2/Rsignal. Hence,the sharpness improvement signal cannot have an amplitude which is lessthan that of the signal VIDEO_IN's active component converted intocurrent ΔV/Rtrans, this amounting to black level clipping (ΔV=0).

The fact that the bias currents Imax vary as a function of the activecomponent ΔV of the signal VIDEO_IN does not affect the sharpnessimprovement signal itself, outside of course of the black level clippingthat is sought. Specifically, this signal corresponds to the differencebetween the currents DOC22 and DOC21, and this difference is notaffected by the variations in the currents Imax, except as regards theclipping.

It will be noted that problems of compensation of the offset between thetransistors of the differential pairs and of speed of the control of thesources of bias current of these differential pairs are resolved by anymeans appropriate on behalf of the person skilled in the art.

It will also be noted that the level of the clipping depends of courseon the improvement in contrast and brightness. It is for this reasonthat the current mirror arrangement generating the commands for thesources SC21 and SC22 additively integrates differential currentsemanating from the stages 30 and 40 in addition to stage 10.

1. A device for producing a sharpness improvement signal for a videoimage while providing black level clipping of an associated videosignal, said device comprising: a first differential transconductancestage having: a voltage input for receiving the video signal, said videosignal having a determined fixed component which corresponds to theblack level and a variable active component; a differential pair with afirst transistor having a control terminal coupled to the voltage input,and a second transistor having a control terminal coupled to the voltageinput via an integrator; a first controlled current source coupled to afirst main terminal of the first transistor and a second controlledcurrent source coupled to a first main terminal of the secondtransistor, said first current source and said second current sourceeach delivering one and the same variable bias current a resistorconnected between said first respective main terminals of the first andof the second transistor; a first current output coupled to a secondmain terminal of the first transistor and a second current outputcoupled to a second main terminal of the second transistor, thesharpness improvement signal being the differential current signalbetween said first and second current outputs; and means for producing acontrol signal for the first and for the second current source, in sucha way that the bias currents are directly proportional to the activecomponent of the video signal.
 2. Device according to claim 1, furthercomprising an input for receiving a control signal for the maximumamplitude or for the time constant of a sharpness peak, by actingrespectively on the value of the resistor or on the value of thehardware items of the integrator.
 3. Device according to claim 1,wherein the means for producing a control signal comprise a differentialtransconductance stage comprising: a second differentialtransconductance stage comprising: a voltage input for receiving thevideo signal; a first differential pair with a first transistor having acontrol terminal coupled to the voltage input, and a second transistorhaving a control terminal receiving a DC voltage corresponding to thefixed component of the video signal, a first controlled current sourcecoupled to a first main terminal of the first transistor and a secondcontrolled current source coupled to a first main terminal of the secondtransistor, said first current source and said second current sourceeach delivering one and the same fixed bias current, a resistorconnected between said first respective main terminals of the first andof the second transistor; a first current output coupled to a secondmain terminal of the first transistor and a second current outputcoupled to a second main terminal of the second transistor, the controlsignal for the first and for the second current source of the firstdifferential transconductance stage being such that the bias current ofthe first differential transconductance stage is equal to half of thedifferential current signal between said first and second currentoutputs.
 4. Device according to claim 3, wherein the second differentialtransconductance stage further comprises: a second differential pairwith a first transistor having a control terminal coupled to the voltageinput, and a second transistor having a control terminal receiving a DCvoltage corresponding to the fixed component of the video signal, afirst main terminal of the first transistor of the first pair, and afirst main terminal of the second transistor being coupled to the firstmain terminal of the second transistor of the first pair, a thirdcurrent output coupled to a second main terminal of said firsttransistor and a fourth current output being coupled to a second mainterminal of said second transistor, the sharpness improvement signalbeing adapted for being added to the differential current signal betweensaid third and fourth current outputs.
 5. Device according to claim 4,wherein the differential currents between the first and second currentoutputs on the one hand, and between the third and fourth currentoutputs on the other hand, have different respective maximum amplitudes.6. Device according to claim 4, wherein the first differentialtransconductance stage and the second differential transconductancestage are coupled by a current mirror arrangement receiving as input thedifferential current signal between said first and second currentoutputs of the first differential pair of the second differentialtransconductance stage, and delivering as output the control signal forthe first and for the second current source of the first differentialtransconductance stage.
 7. Device according to claim 6, wherein thecurrent mirror arrangement is included in the first differentialtransconductance stage.
 8. A video preamplification circuit comprising adevice according to claim
 1. 9. A device comprising: a video inputstage, a signal adjuster coupled to the video input stage, a switchcoupled to the signal adjustor and to the video input stage; and a videooutput stage coupled to the switch.
 10. The device of claim 9 furthercomprising a system for displaying the video signal, said system coupledto the video signal output.
 11. The device of claim 9 where the signaladjuster comprises independent elements for adjusting the sharpness,brightness, and contrast of a video signal.
 12. A method for adjusting avideo image comprising: receiving a video signal, adjusting the qualityof the video signal dependant upon which screen zone is to be displayed.13. The method of claim 12 where the adjustment comprises: changing thesharpness of the signal to be dispalyed, changing the brightness of thesignal to be displayed, changing the contrast of the signal to bedisplayed.
 14. The method according to claim 12 where the adjusted videosignal is displayed on a CRT, LCD, or similar device.
 15. The methodaccording to claim 12 wherein the steps to produce the device foradjusting a video signal comprise: constructing one or more stages toadjust the video signal, connecting the adjusting stages to a videoinput, connecting the output of the adjusting stages to a switch,connecting the video signal to the alternate pole of the switch,connecting an image control signal to the switch selector.